AP Biology Reading Guide Fred and Theresa Holtzclaw
Chapter 23: The Evolution of Populations
Name _______________________ Period ___________
Chapter 23: The Evolution of Populations This chapter begins with the idea that we focused on as we closed the last chapter: Individuals do not evolve! Populations evolve. The Overview looks at the work of Peter and Rosemary Grant with Galápagos finches to illustrate this point, and the rest of the chapter examines the change in populations over time. As in the last chapter, first read each concept to get the big picture and then go back to work on the details presented by our questions. Don’t lose sight of the conceptual understanding by getting lost in the details! Overview 1.
What is microevolution?
2.
What are the three main mechanisms that can cause changes in allele frequency?
3.
Which is the only mechanism that is adaptive, or improves the match between organisms and their environment?
Concept 23.1 Mutation and sexual reproduction produce the genetic variation that makes evolution possible 4.
Because Darwin did not know about the work of Gregor Mendel, he could not explain how organisms pass heritable traits to their offspring. In looking at genetic variation, what are discrete characters, and what are quantitative characters?
5.
Using the techniques of molecular biology, what are the two ways of measuring genetic variation in a population?
6.
Geographic variation may be shown in a graded manner along a geographic axis known as a cline. What external factors might produce a cline? Why does the existence of a cline suggest natural selection?
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AP Biology Reading Guide Fred and Theresa Holtzclaw
Chapter 23: The Evolution of Populations
7.
What is the ultimate source of new alleles?
8.
Mutations are any change in the nucleotide sequence of an organism’s DNA. These mutations provide the raw material from which new traits may arise and be selected. What occurs in a point mutation?
9.
What is translocation? How could it be beneficial?
10.
How does gene duplication occur? How might it play a role in evolution?
11.
Much of the genetic variation that makes evolution possible comes through sexual reproduction. What are the three mechanisms by which sexual reproduction shuffles existing alleles?
Concept 23.2 The Hardy-Weinberg equation can be used to test whether a population is evolving 12.
What is a population?
13.
What is a gene pool?
14.
The greater the number of fixed alleles, the lower the species’ diversity. What does it mean to say that an allele is fixed?
15.
The Hardy-Weinberg principle is used to describe a population that is not evolving.What does this principle state?
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AP Biology Reading Guide Fred and Theresa Holtzclaw
16.
Chapter 23: The Evolution of Populations
If the frequency of alleles in a population remains constant, the population is at HardyWeinberg equilibrium. There are five conditions for Hardy-Weinberg equilibrium. It is very important for you to know these conditions, so enter them neatly into the box below. CONDITIONS FOR HARDY-WEINBERG EQUILIBRIUM
1. 2. 3. 4. 5.
It is not very likely that all five of these conditions will occur, is it? Allelic frequencies change. Populations evolve. This data can be tested by applying the Hardy Weinberg equation. Let’s look at how to do this. Equation for Hardy-Weinberg Equilibrium p2 + 2pq + q2 = 1 Where p2 is equal to the frequency of the homozygous dominant in the population, 2pq is equal to the frequency of all the heterozygotes in the population, and q2 is equal to the frequency of the homozygous recessive in the population. Consider a gene locus that exists in two allelic forms, A and a, in a population. Let p = the frequency of A, the dominant allele and q = the frequency of a, the recessive allele. So, p2 = AA, q2 = aa, 2pq = Aa If we know the frequency of one of the alleles, we can calculate the frequency of the other allele: p + q = 1, so p=1–q q=1–p
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AP Biology Reading Guide Fred and Theresa Holtzclaw
Chapter 23: The Evolution of Populations
17.
So, here is a problem to try. Suppose in a plant population that red flowers (R) are dominant to white flowers (r). In a population of 500 individuals, 25% show the recessive phenotype. How many individuals would you expect to be homozygous dominant and heterozygous for this trait? (A complete solution for this problem is at the end of this Reading Guide.)
18.
In a population of plants, 64% exhibit the dominant flower color (red), and 36% of the plants have white flowers. What is the frequency of the dominant allele? (There are a couple of twists in this problem, so read and think carefully. A complete solution for this problem is at the end of this Reading Guide.)
Concept 23.3 Natural selection, genetic drift, and gene flow can alter allele frequencies in a population 19.
Factor
First, let’s try to summarize the big idea from this section. Scan through the entire concept to pull out this information. Three major factors alter allelic frequency and bring about evolutionary change. List each factor, and give an explanation. Explanation
20.
Which of the factors above results in a random, nonadaptive change in allelic frequencies?
21.
Which of the factors above tends to reduce the genetic differences between populations and make populations more similar?
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AP Biology Reading Guide Fred and Theresa Holtzclaw
Chapter 23: The Evolution of Populations
22.
Of the three factors you listed above, only one results in individuals that are better suited to their environment. Which is it?
23.
Explain what happens in each of these examples of genetic drift: founder effect bottleneck effect
Concept 23.4 Natural selection is the only mechanism that consistently causes adaptive evolution 24.
In evolutionary terms, fitness refers only to the ability to leave offspring and contribute to the gene pool of the next generation. It may have nothing to do with being big, or strong, or aggressive. Define relative fitness.
25.
What is the relative fitness of a sterile mule? _______________________________________
26.
Figure 23.13 is important because it helps explain the three modes of selection. Label each type of selection, and fill in the chart to explain what is occurring.
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AP Biology Reading Guide Fred and Theresa Holtzclaw Type of Selection Stabilizing
Chapter 23: The Evolution of Populations
How It Works
Directional
Disruptive
27.
What is often the result of sexual selection?
28.
What is the difference between intrasexual selection and intersexual selection? Give an example of each type of selection.
29.
Explain two ways in which genetic variation is preserved in a population.
30.
Discuss what is meant by heterozygote advantage, and use sickle-cell anemia as an example.
31.
Finally, give four reasons why natural selection cannot produce perfect organisms.
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AP Biology Reading Guide Fred and Theresa Holtzclaw
Chapter 23: The Evolution of Populations
Testing Your Knowledge: Self-Quiz Answers Now you should be ready to test your knowledge. Place your answers here: 1. __________2.__________3. __________4. __________5. __________
Solution to Question 17 Let p = frequency of the dominant allele (R) and q = frequency of the recessive allele (r). 1. q2 = frequency of the homozygous recessive = 25% = 0.25. Since q2 = 0.25, q = 0.5. 2. Now, p + q =1, so p = 0.5. 3. Homozygous dominant individuals are RR or p2 = 0.25, and they will represent (0.25)(500) = 125 individuals. 4. The heterozygous individuals are calculated from 2pq = (2)(0.5)(0.5) = 0.5, and in a population of 500 individuals will be (0.5)(500) = 250 individuals.
Solution to Question 18 This problem requires you to recognize that individuals with the dominant trait can be either homozygous or heterozygous. Therefore, you cannot simply take the square root of 0.64 to get p. For problems of this type, you must begin with the homozygous recessive group. So . . . Let p = frequency of the dominant allele (R) and q = frequency of the recessive allele (r) 1. q2 = frequency of the homozygous recessive = 36% = 0.36. Because q2 = 0.36, q = 0.6. 2. Now, p + q =1, so p = 0.4. 3.
Notice that this problem asks for the frequency of the dominant allele (p), not the frequency of the homozygous dominant individuals (p2). So, you are done . . . the frequency of the dominant allele = 40%.
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